Various means have been used in measuring displacement, such as methods used in determining strains in solids. One such system is described in the patent to Bell, U.S. Pat. No. 2,929,242 wherein a diffraction grating is used to measure strain. The patent to Pryor, U.S. Pat. No. 3,664,739 discloses a system for measuring the displacement of the edges of a slit with the use of a diffraction pattern. It is a well known fact of optics that monochromatic, coherent light passing through two narrow slits separated by a small distance will produce an interference pattern with the spacing of the interference fringes being proportional to the distance between the slits. In the measurement of displacement between positions on objects or relative movement between pairs of objects, it is sometimes difficult to provide slits to produce the interference pattern.